Helical optical pulse stretcher

ABSTRACT

A pulse stretcher includes a single pass pulse stretcher. An optical system is arranged around the single pass pulse stretcher. A beam enters the single pass stretcher and is reflected in a helical path using the optical system for multiple passes through the single pass pulse stretcher. The single pass pulse stretcher can include two 90° prisms, with a beam splitter located therebetween. The optical system can include first and second prisms. At least one of the first and second prisms can be a roof prism. The first and second prisms can have at least one surface oriented so as to direct the beam into the helical path. The optical system can have at least one mirror, or a plurality of mirrors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pulse stretchers, and moreparticularly, to helical pulse stretchers for use in lithographyapplications.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays (e.g., liquid crystal displays), circuit boards,various integrated circuits, and the like. A frequently used substratefor such applications is a semiconductor wafer or glass substrate.

During lithography, a wafer, which is disposed on a wafer stage, isexposed to an image projected onto the surface of the wafer by exposureoptics located within a lithography apparatus. While exposure optics areused in the case of photolithography, a different type of exposureapparatus can be used depending on the particular application. Forexample, x-ray, ion, electron, or photon lithography each can require adifferent exposure apparatus, as is known to those skilled in the art.The particular example of photolithography is discussed here forillustrative purposes only.

The projected image produces changes in the characteristics of a layer,for example photoresist, deposited on the surface of the wafer. Thesechanges correspond to the features projected onto the wafer duringexposure. Subsequent to exposure, the layer can be etched to produce apatterned layer. The pattern corresponds to those features projectedonto the wafer during exposure. This patterned layer is then used toremove or further process exposed portions of underlying structurallayers within the wafer, such as conductive, semiconductive, orinsulative layers. This process is then repeated, together with othersteps, until the desired features have been formed on the surface, or invarious layers, of the wafer.

A difficulty in designing optical components used in microlithography iscaused by the high power density involved during exposure. High-poweredlasers are often used as a source of illumination in microlithographicsystems. These lasers are usually pulsed lasers, which generaterelatively short high intensity pulses. The high intensity pulses, overtime, damage optical components, such as lenses used in the projectionoptics, requiring frequent replacement of these optical components. Toreduce the effect of the high power density, pulse stretchers are oftenused in lithographic equipment. A pulse stretcher is a passive opticalcomponent that uses an arrangement of beam splitters to extend theduration of the pulse. Pulse stretchers can sometimes be cascaded, tomultiply the effect of a single pulse stretcher. However, the volume ofspace available for the projection optical system with the lithographictool is limited. Consequently, the volume of space available for thepulse stretcher is also limited, often precluding the use of a largenumber of cascaded pulse stretchers.

Thus, the purpose of the pulse stretcher is to maintain the energy ofthe pulse generally constant, while reducing peak intensities byspreading out the pulse in time.

Accordingly, what is needed is a pulse stretcher that is compact andpermits a significant stretching of the pulse from a lithographicillumination source.

SUMMARY OF THE INVENTION

The present invention is directed to a helical optical pulse stretcherthat substantially obviates one or more of the problems anddisadvantages of the related art.

The present invention, in one embodiment, includes a pulse stretcherincluding a single pass pulse stretcher. An optical system is arrangedaround the single pass pulse stretcher. A beam enters the single passstretcher and is reflected in a helical path using the optical systemfor multiple passes through the single pass pulse stretcher. The singlepass pulse stretcher can include two 90° prisms, with a beam splitterlocated therebetween. The optical system can include first and secondprisms. At least one of the first and second prisms can be a roof prism.The first and second prisms can have at least one surface oriented so asto direct the beam into the helical path. The optical system can haveone or more mirrors.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIGS. 1A and 1B show an exemplary embodiment of the present invention.

FIG. 2 is a plot illustrating an effect of using the pulse stretcher ofthe present invention.

FIG. 3 shows a lithography system according to an embodiment of thepresent invention.

FIG. 4 shows an alternative embodiment using a plate beamsplitter.

FIGS. 5-9 show alternative embodiments using different mirrorcombinations.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

FIGS. 1A and 1B show an exemplary embodiment of the present invention.For clarity, FIG. 1A shows only the optical components, while FIG. 1Bshows the same components with the propagating light beams added andlabeled. In the discussion below, the sense of direction (i.e., left,right, up, down) is for illustrative use only, and is not intended tolimit the present invention.

FIG. 1A illustrates a perspective view of an exemplary pulse stretcherof the present invention. Shown in FIG. 1A are the following components:a prism 102, a beam splitter 110, a prism 104, and two prisms 106 and108. Prisms 102 and 104 are usually 90° prisms, in other words, as shownby the heavy lines in FIG. 1B, an input beam 111 enters from the leftand exits from the right at beam 130L. The input beam 111 travels closeto (but does not enter) the prism 106. Ultimately, the output beam 130Lis a “stretched” pulse (in the sense of full width half maximum)compared to the pulse of the input beam 111.

As shown in FIG. 1B, the beam 111 enters from the left. Approximatelyhalf of the beam 111 continues straight (see 112), while the other halfof the beam 111 is reflected by a beam splitter upwards (see beam 122),then is reflected off of surface 102A (beam 124), then is reflected offof surface 102B as beam 116. (Note that surfaces 102A, 102B act asreflective surfaces in this case.) On the return path, the beam 120encounters the beamsplitter 110. Half of the beam 120 is reflected tothe right by the beamsplitter 110 (combining with beam 112), and halfcontinues upwards through the beamsplitter 110 (see beam 122). Thus,considering for the moment only the “thick” beam path in FIG. 1B, thispath represents a single-pass beam splitter-pulse stretcher. Theexpansion of the pulse duration is proportional to the amount of timethat it takes for the beam to go “around the loop” (represented by thethick dashed lines). This corresponds to the delay introduced by thesystem of the prisms 102, 104 and the beam splitter 110.

In practical applications, the amount of delay possible depends onphysical system parameters. Greater pulse stretching means longer pathlengths for the beams 116 and 120 in FIG. 1B. Such longer path lengthsmay, to some extent, be achieved using folded optical paths, e.g., usingmirrors to fold the optical axis. However, the addition of extrasurfaces adds to system complexity, particularly since, to achieve asubstantial pulse stretching effect, the effective optical path lengthof the beams 116 and 120 needs to be fairly long, compared to dimensionsof the optical components. This means that the optical path needs to befolded, not just in two dimensions, but often in three dimensions. Thisintroduces additional alignment difficulties.

The prisms 108 and 106 are used to recirculate the beam back through the102, 104 and 110 components multiple times. As may be further seen inFIGS. 1A and 1B, prism 108 has surfaces 108A and 108B. At least one (andoptionally both) surfaces 108A and 108B are oriented such that afterreflections of the surfaces, the beam 130 (in this case, 130A, 130E,1301, see FIG. 1B) is displaced slightly in the direction out of thepaper in FIG. 1B. Similarly, the prism 106 has surfaces 106A and 106B,at least one of which is also oriented at a slight angle so as todisplace the beam slightly in the direction out of the page. The resultof this is a beam that travels along a helical path, in this case issegments 130A-130L, until finally exiting out of the pulse stretcher(beam 130L). Note that since beams used in optical lithographicequipment are often rectangular, the pulse stretcher shown in FIG. 1A isparticularly useful in that application. Each pass of the beam as itgoes through the components 102, 110, 104 results in 50% of the lightbeing “stretched.” Thus, the effect is like having cascaded, or tandem,pulse stretchers.

Note that instead of prisms 106, 108, which are usually roof-typeprisms, mirror surfaces can be used. Alternatively, only some of thesurfaces may be used as mirrors, e.g., only prism 108, or only prism 106can be replaced with mirrors, while the other prism remains as describedabove. Note also that the beam splitter 110 is shown as a cube, althoughdoes not need to be cubical, since any other form of beam splitters maybe used (see, e.g., plate beamsplitter 401 shown in an alternativeembodiment in FIG. 4).

As a practical matter, a single pulse stretcher with a long delay doesnot be as efficient as stretching the pulse as using a number of pulsestretchers arranged in a cascade manner. The structure shown in FIG. 1Aallows the use of a single pulse stretcher, while achieving theperformance of multiple pulse stretchers arranged in a cascade.

Note further that although the prisms 106 and 108 are shown asidentical, this need not be the case. The only requirement is that theprisms 106 and 108B be arranged opposite to each other, and at an angleto the path of the incoming beam (i.e. parallel to the beam 111). Thisenables the helical path of the beam 130A-130L.

In a typical configuration, the number of passes to the prisms 106, 108would be between 2 and 5, although more passes are possible. The opticaldesigner has to balance the need for more effective pulse stretchingagainst the losses in the optics. Thus, roughly three passes is believedto be optimal.

The size of the components shown in FIG. 1A depends on the dimensions ofthe beam. If the beam were a “pencil” beam of about 1 mm×1 mm, such asshown schematically in FIG. 1A, then the components could be quitesmall. However, many practical systems have larger rectangular beams, inthe order of 25 millimeters by 5 millimeters. In that case, “out of thepage” in FIG. 1A would be preferably chosen as the short direction. Theprism dimensions would then be on the order of 50 millimeters. Note alsothat prisms 102, 104 can also be replaced by two mirrored surfaces each(in other words, instead of the prism 104, mirrors can be placed wherethe surfaces 104A, 104B, or where the surfaces 102A, 102B are located(see, e.g., plate beamsplitter 502, 504, 506, 508 shown in analternative embodiment in FIG. 5, and mirrors 602, 604, 606, 608 shownin an alternative embodiment in FIG. 6). The prisms maybe made ofoptical glass, quartz, fused silica, etc., as required by the wavelengthof the source. Powered surfaces may be used as prism or mirror surfaces,to counteract beam dispersion. Also, lens(es) may be inserted into thebeam path, to counteract beam dispersion.

FIG. 2 shows an effect of using the pulse stretcher of the presentinvention on the pulse profile. The horizontal axis is time (innanoseconds), and the vertical axis is power density of the pulse, inrelative units. Note that the pulse, after passing through the pulsestretcher of the present invention, is much smoother and longer, withcorrespondingly lower power densities.

Note further that elements 102, 110, 104 need not be the same as shownin FIG. 1A. Any single-pass pulse stretcher may be placed “in themiddle” between the prisms 106, 108 to achieve the same effect (withproper alignment). Note further that it is also possible to replace theprisms 106, 108 with three mirrors instead of four, see mirrors 602,604, 608 illustrated in an alternative embodiment in FIG. 7 (althoughthe angles would change, compared to what is shown in FIG. 1A). As yetanother alternative, it is possible to use multiple mirrors arranged asshown, for example, by mirrors 802, 804, 806, 808, 810 and 812 in FIG.8.

It is also possible to use only one mirror each, to replace prisms 106,108, as shown by elements 902, 904 in an alternative embodiment of FIG.9. In that case, it would not be possible to have a helical path asshown by the beam path 130A-130L and the price paid for that is a slightblurring of the beam in at least one dimension. However, it is possibleto accomplish the pulse stretching, if the engineering consequences ofthe above are accepted, with using only two mirrors to replace prisms106, 108. Note that in a case of replacing only a single of the roadprisms 106, 108 with a single mirror, the remaining roof prism would notbe a 45° prism but a slightly different angle, in order to achieve thesame effect.

There are a number of ways of manufacturing the pulse stretcher shown inFIG. 1A. For example, it is possible to manufacture all the componentsas separate components, and then align them together. It is possible tobond the beam splitter 110 to the prism 102. It is also possible tomanufacture, for example, components 106, 110 and 102 as a singlecomponent or components 106, 102 110 and 108 as a single component, orsome combination thereof. Because the relative angular alignment betweenthe shown surfaces is important, and because manufacturing singlecomponents with the required tolerances and angular orientation can bemore difficult than manufacturing them separately, and then aligningthem relative to each other, there are engineering tradeoffs betweenmanufacturability and subsequent alignment.

FIG. 3 shows a system 300 according to an embodiment of the presentinvention. System 300 includes an illumination source 302 that outputslight to illumination optics 304. Illumination optics 304 direct thelight through (or off) a mask or reticle 306 onto a substrate 308 viaprojection optics 310. The pulse stretcher is shown as element 320 inFIG. 3. One embodiment for this system can be a lithography system, orthe like. Another embodiment can be a holography system.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A pulse stretcher comprising: a single pass pulse stretcher; and a first prism and a second prism arranged on either side of the single pass pulse stretcher, wherein a beam enters the single pass stretcher and is reflected in a helical path using the first and second prisms for multiple passes through the single pass pulse stretcher.
 2. The pulse stretcher of claim 1, wherein the single pass pulse stretcher includes two 90° prisms with a beam splitter located therebetween.
 3. The pulse stretcher of claim 1, wherein first and second prisms are roof prisms.
 4. The pulse stretcher of claim 1, wherein the first and second prisms have at least one surface oriented so as to direct the beam into the helical path.
 5. A pulse stretcher comprising: a single pass pulse stretcher; an optical system arranged on either side of the single pass pulse stretcher, wherein a beam enters the single pass stretcher and is reflected in a helical path using the optical system for multiple passes through the single pass pulse stretcher.
 6. The pulse stretcher of claim 5, wherein the single pass pulse stretcher includes two 900 prisms with a beam splitter located therebetween.
 7. The pulse stretcher of claim 5, wherein the optical system includes first and second prisms.
 8. The pulse stretcher of claim 7, at least one of the first and second prisms is a roof prism.
 9. The pulse stretcher of claim 7, wherein the first and second prisms have at least one surface oriented so as to direct the beam into the helical path.
 10. The pulse stretcher of claim 5, wherein the optical system includes at least one mirror.
 11. The pulse stretcher of claim 5, wherein the optical system includes a plurality of mirrors.
 12. A lithographic system comprising, in order of light propagation: an illumination source; a pulse stretcher; illumination optics; a reticle; and projection optics wherein the pulse stretcher comprises: a single pass pulse stretcher; a first prism and a second prism arranged on either side of the single pass pulse stretcher, wherein a beam enters the single pass stretcher and is reflected in a helical path using the first and second prisms for multiple passes through the single pass pulse stretcher.
 13. The lithographic system of claim 12, wherein the single pass pulse stretcher includes two 90° prisms with a beam splitter located therebetween.
 14. The lithographic system of claim 12, wherein first and second prisms are roof prisms.
 15. The lithographic system of claim 12, wherein the first and second prisms have at least one surface oriented so as to direct the beam into the helical path.
 16. A lithographic system comprising, in order of light propagation: an illumination source; a pulse stretcher; illumination optics; a reticle; and projection optics wherein the pulse stretcher comprises: a single pass pulse stretcher; and an optical system arranged on either side of the single pass pulse stretcher, wherein a beam enters the single pass stretcher and is reflected in a helical path using the optical system for multiple passes through the single pass pulse stretcher.
 17. The lithographic system of claim 16, wherein the single pass pulse stretcher includes two 90° prisms with a beam splitter located therebetween.
 18. The lithographic system of claim 16, wherein the optical system includes first and second prisms.
 19. The lithographic system of claim 18, at least one of the first and second prisms is a roof prism.
 20. The lithographic system of claim 18, wherein the first and second prisms have at least one surface oriented so as to direct the beam into the helical path.
 21. The lithographic system of claim 16, wherein the optical system includes at least one mirror.
 22. The lithographic system of claim 16, wherein the optical system includes a plurality of mirrors. 